Conventionally, a radio frequency power amplifier for amplifying a modulated signal with a variable envelope uses a class A or class AB linear amplifier for linearly amplifying the variable envelope. Such a linear amplifier provides a superb linearity, but constantly consumes power accompanying a DC bias component and thus has a lower power efficiency than, for example, class C through class E nonlinear amplifiers. Therefore, when applied to a mobile communication device using a battery as a power source, such a radio frequency power amplifier has a problem of being usable for only a short period of time due to the high power consumption thereof. When applied to a base station device of a wireless system including a plurality of high power transmission circuits, such a radio frequency power amplifier has a problem of enlarging the device and increasing the power dissipation.
In light of these problems, a transmission circuit using a polar modulation method has been proposed as a transmission circuit operable at a high efficiency. FIG. 24 is a block diagram showing a structure of a conventional transmission circuit 500 using the polar modulation method. As shown in FIG. 24, the conventional transmission circuit 500 includes a signal generation section 501, an angle modulation section 502, a power source terminal 503, a regulator 504, an amplitude modulation section 505, and an output terminal 506.
The signal generation section 501 generates an amplitude signal and a phase signal. The amplitude signal is input to the regulator 504. The regulator 504 is supplied with a DC voltage from the power source terminal 503. The regulator 504 supplies a voltage corresponding to the input amplitude signal to the amplitude modulation section 505. The phase signal is input to the angle modulation section 502. The angle modulation section 502 performs angle modulation on the input phase signal and outputs an angle-modulated signal. The angle-modulated signal which is output from the angle modulation section 502 is input to the amplitude modulation section 505. The amplitude modulation section 505 performs amplitude modulation on the angle-modulated signal with the voltage supplied from the regulator 504, and outputs the resultant signal as a modulated signal. This modulated signal is output from the output terminal 506 as a transmission signal. In this way, the transmission circuit 500 can output transmission signals at a high efficiency.
However, a transmission signal which is output from the transmission circuit using the polar modulation method may occasionally distorted by the nonlinear characteristic of the amplitude modulation section 505 or the like. FIG. 25 shows a characteristic of the output power from the amplitude modulation section 505 with respect to the input voltage from the regulator 504. As is clear from FIG. 25, the amplitude modulation section 505 has a nonlinear area and a linear area. In order to obtain a small output power, the amplitude modulation section 505 needs to operate in the nonlinear area. When the amplitude modulation section 505 operates in such a nonlinear area, the transmission signal is undesirably distorted.
A technique for compensating for the nonlinearity of the amplitude modulation section 505 or the like in a transmission circuit using the polar modulation method has been disclosed (see, for example, U.S. Pat. No. 6,366,177). One conventional transmission circuit using such a technique is, for example, a transmission circuit 600 shown in FIG. 26. FIG. 26 is a block diagram showing an exemplary structure of the conventional transmission circuit 600. Referring to FIG. 26, a predistortion section 601 creates a predistortion table for compensating for the nonlinearity of the amplitude modulation section 505 based on a transmission signal. Then, based on the predistortion table created by the predistortion section 601, an amplitude control section 602 and a phase control section 603 respectively pre-distort an amplitude signal and a phase signal, and input the resultant signals to the regulator 504 and the amplitude modulation section 505. In this way, the transmission circuit 600 can compensate for the nonlinearity of the amplitude modulation section 505 or the like.
For a transmission circuit using the polar modulation method, a technique for putting a limitation on a small-amplitude component of an amplitude signal and thus suppressing the amplitude modulation section 505 from operating in the nonlinear area has also been proposed (see, for example, Japanese Laid-Open Patent Publication No. 2005-45782). One conventional transmission circuit using such a technique is, for example, a transmission circuit 700 shown in FIG. 27. FIG. 27 is a block diagram showing an exemplary structure of the conventional transmission circuit 700. Referring to FIG. 27, when the magnitude of an amplitude signal becomes smaller than a predetermined threshold value, an amplitude limiting section 701 shapes the waveform of the amplitude signal such that the magnitude of such a small-magnitude part of the amplitude signal is raised to the predetermined magnitude. In this way, the transmission circuit 700 can operate the amplitude modulation section 505 in the linear area even when the magnitude of the amplitude signal becomes smaller than the predetermined threshold value.
With the transmission circuit 600 shown in FIG. 26, it is difficult to compensate for the nonlinearity of the amplitude modulation section 505 because the nonlinearity easily changes in accordance with the temperature. Even when the input power to the amplitude modulation section 505 is made sufficiently low, an output signal having a power equal to or lower than a predetermined level cannot be obtained. For these reasons, the transmission circuit 600 has a problem that the nonlinearity of the amplitude modulation section 505 cannot be compensated for when the temperature of the amplitude modulation section 505 changes or when the power to be output is very low.
With the transmission circuit 700 shown in FIG. 27, a limitation is put on a small-amplitude component of the amplitude signal when the magnitude of the amplitude signal becomes smaller than the predetermined threshold value. The transmission circuit 700 has a problem that a transmission signal is distorted by putting such a limitation.
Therefore, an object of the present invention is to provide a transmission circuit for outputting transmission signals with a low distortion and a high efficiency over a wide range of output power, and a communication device using the same.